1. Field of the Invention
The present invention relates to an electronic device and a light emission control method for the electronic device and, in particular, to: an electronic device having a light emitting section that emits light by utilizing recombination of electrons and holes; and a light emission control method for this electronic device.
2. Description of the Related Art
Recently, as the light source of a floodlighting device, a lighting device, and the like, an electronic device is used more frequently that has a light emitting section that emits light using a P-N junction of a semiconductor. In particular, an electronic device is also known that employs a light emission structure in which an active layer such as a quantum well active layer is provided between a P-type semiconductor layer and an N-type semiconductor layer that constitute a P-N junction so that light emission is performed effectively.
In an electronic device having such a light emitting section, when a required driving signal is inputted to the light emitting section, carriers consisting of free electrons and free holes recombine with each other in the P-N junction region or the active layer of the light emitting section so that the light emission phenomenon is generated.
As known, in the light emitting section, at the time of light emission, Joule heat is generated by the resistance components of the semiconductor layer part such as the P-type semiconductor layer, the N-type semiconductor layer, and the active layer. Then, the Joule heat generates defects in the crystal structure of the semiconductor layer so as to reduce the optical output power of the light emitting section as time advances.
Further, according to the findings of the present inventors, each defect level present in the semiconductor layer part of a light emitting section captures an electron and a hole and then causes them to recombine with each other. Then, recombination energy released in accordance with this recombination is released as heat energy. This heat energy then causes multiplication and diffusion of defect levels in the semiconductor layer so as to degrade the semiconductor layer.
Then, when the semiconductor layer is degraded, the efficiency of recombination of electrons and holes decreases in the P-N junction region or the active layer, and hence the luminance of light emission decreases. Thus, in such a light emitting section and an electronic device having such a light emitting section, when the luminance of the light emitting section goes to or below a desired luminance, it is determined that the product lifetime has been reached and hence the light emitting section needs to be replaced.
A longer lifetime of the light emitting section is more preferable. Thus, in the prior art, enhancement of the lifetime of the light emitting section has been achieved by reducing the density of the defect levels itself generated in the semiconductor layer part of the light emitting section at a manufacturing stage.
Alternatively, according to a proposal having been made so far, focusing attention on Joule heat, a pulsed driving signal is inputted to a light emitting section so that the light emitting section is brought into an ON state and an OFF state alternately so as to emit light intermittently. By virtue of this, energization duration per unit time is reduced, and hence heat generation is reduced in the light emitting section. As a result, lifetime improvement is achieved (see, for example, Patent Document 1).
A detailed method is as follows. When a temperature increase that occurs in the light emitting section in a case that continuous energization is performed on the light emitting section so that continuous light emission is performed is denoted by ΔT0 while a temperature increase that occurs in the light emitting section in a case that a pulsed driving signal is inputted to the light emitting section so that intermittent light emission is performed is denoted by ΔT1, a pulsed driving signal is adopted that has a pulse width and a duty ratio satisfying a condition ΔT1/ΔT0<1/2 (see, for example, Patent Document 2).
Further, according to another proposal, when a semiconductor laser is employed which has a threshold current I0 and a slope efficiency η and in which an injection current dependence of the lifetime in continuous light emission is approximated as τ(I)=cI−r (here, I is an injection current and c is a constant) in an output area of interest, an output P satisfying{(I0+P/η)/(I0+P0/η)}r<P/P0 
for a required average output P0 is adopted as well as a duty ratio β=P0/P (see, for example, Patent Document 3).
[Patent Document 1] International Publication No. 2009/057561 Pamphlet
[Patent Document 2] Japanese Published Unexamined Patent Application No. H09-052389
[Patent Document 3] Japanese Published Unexamined Patent Application No. 2000-133873
Nevertheless, according to the proposed duty ratio conditions, in order that the effect of lifetime improvement should be obtained, a duty ratio lower than 0.4 needs to be adopted. When such a low duty ratio is adopted, the apparent luminance of the light emitting section decreases remarkably, and hence this condition is not a practical operating condition. This has been a problem.
That is, in such an electronic device having a light emitting section, in general, operation is performed in such a manner that the light emitting section always emits light so that a higher luminance is obtained. Thus, a duty ratio as close to 1.0 as much as possible is desired, and hence a duty ratio reduced to 0.7 or lower is not practical.
On the other hand, according to the findings of the present inventors, one of the causes degrading the semiconductor layer of the light emitting section is recombination of electrons and holes in defect levels present in the semiconductor layer. Further, this process has a remarkably large influence. Thus, we have recognized that when attention is focused on defect levels, lifetime improvement of the light emitting section can be achieved regardless of the duty ratio.